This was partially implemented in Vanilla Dungeon on the last level, though there was no quest check to make sure only 1 item is dropped. </self-promotion>
A consideration is that it uses an awful lot of events, so you may want to have 'treasure dungeons' just like we have 'text dungeons' now where the party gets teleported to get the item (and then teleported back).
Also note you can only drop 1 item or 1 set of items at a time. (This was actually done in Death Knights of Krynn, where you could try to go into the pit and you'd get a different set of random items each time, but they always went together--they had the longsword +3 with the banded mail +4 if I remember right.)